As will be appreciated herein below, except as otherwise indicated, alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2008 as is well known in the art.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
It is known in the art to use heat treatable aluminium alloys in a number of applications involving relatively high strength, high toughness and corrosion resistance such as aircraft fuselages, vehicular members and other applications. Aluminium alloys AA7050 and AA7150 exhibit high strength in T6-type tempers. The T6 temper is known to enhance the strength of the alloy, wherein the aforementioned AA7050 and AA7x50 alloy products which contain high amounts of zinc, copper and magnesium are known for their high strength-to-weight ratios and, therefore, find application in particular in the aircraft industry. However, these applications result in exposure to a wide variety of climatic conditions necessitating careful control of working and ageing conditions to provide adequate strength and resistance to corrosion, including both stress corrosion and exfoliation. In order to enhance resistance against stress corrosion and exfoliation as well as fracture toughness it is known to artificially over-ageing these 7000-series alloys. When artificially aged to for example a T79, T76, T74 or T73-type temper their resistance to stress corrosion, exfoliation corrosion and fracture toughness improve in the order stated but at some cost to strength compared to the T6 temper condition. An acceptable temper condition is the T74-type temper which is a limited over-aged condition, between T73 and T76, in order to obtain an acceptable level of tensile strength, stress corrosion resistance, exfoliation corrosion resistance and fracture toughness.
However, for thick sectional parts having a thickness of more than about 3 inch or parts machined from such thick sections, a uniform and reliable property balance through thickness is important. Currently, amongst others AA7050 or AA7010 or AA7040 or AA7085 are used for these types of applications. Reduced quench sensitivity, that is deterioration of properties through thickness with lower quenching speed or thicker products, is a major wish from amongst others the aircraft manufactures.
In the production of this type of alloy wrought products these are commonly subjected to a solution heat treatment followed by quenching. In solution heat treating and quenching thick sections, the quench sensitivity of the alloy product is of great concern. After solution heat treating, it is desirable to quickly cool the product for retaining various alloying elements in solid solution rather than allowing them to precipitate out of solution in coarse form as otherwise occurs via slow cooling. The latter occurrence produces coarse precipitates, e.g. Al2CuMg and/or Mg2Zn, and results in a decline in mechanical properties. In products with thick cross sections, the quenching medium acting on exterior surfaces of such products (either plate, extrusion or forging) cannot efficiently extract heat from the interior including the centre or mid-plane or quarter-plane of that material. This is due to the physical distance to the surface and the fact that heat extracts through the metal by a distance dependent conduction. In thin cross sections (e.g. 2 inch or less), quench rates at the mid-plane are naturally higher than quench rates for a thicker product cross sections. Hence, an alloy product's overall quench sensitivity is often not as important in thinner gauges as it is for thicker gauged products, at least from the standpoint of strength and toughness.
U.S. Pat. No. 6,027,582, forming the basis for the AA7040 development, discloses an optimised balance between alloying elements for improving strength and other properties while avoiding excess additions to minimize quench sensitivity.
US patent application US-2002/0121319-A1, forming the basis for the AA7085 alloy development, discloses another carefully controlled balance of the addition of Zn, Mg and Cu to provide an improved quench sensitivity while maintaining good strength-toughness properties, in particular in thicker gauge aluminium products.
US patent application US-2006/0096676 discloses another controlled 7xxx-series alloy product having high Mg content of 2.6 to 3.0% Mg, a very low Cu-content of 0.10 to 0.2% and a purposive addition of 0.05 to 0.2% Zr to achieve a fine grain structure in the plate product by selecting a combined homogenisation and solution heat treatment with subsequent two-stage cooling to reduce the quench sensitivity in the plate product.
Some other prior art references are:
Japanese patent application JP-10-212538-A discloses a thin gauge aluminium alloy clad product for heat exchangers. The product comprises of an aluminium alloy core layer having an aluminium alloy cladding layer comprises 0.005 to 2.0% of Ge to suppress the formation of oxidized coating on the surface of the sacrificial material in an alkaline environment. The cladding layer preferably further comprises at least 0.1 to 6% Zn, 0.1 to 3.55% Mg. In addition 0.005 to 0.5% of In or Sn may be added as these have a similar effect as Zn. Also V may be added, as well as Si in a range of 0.1 to 0.7% to improve the strength.
International patent application WO-2004/090185 discloses an aluminium alloy product with high strength and fracture toughness and a good corrosion resistance, said alloy comprising essentially, in wt. %: Zn 6.5 to 9.5, Mg 1.2 to 2.2, Cu 1.0 to 1.9, Fe<0.3, Si<0.20, optionally one or more of: (Zr<0.5, Sc<0.7, Cr<0.4, Hf<0.3, Mn<0.8, Ti<0.4, V<0.4), and other impurities or incidental elements and the balance being aluminium. It is disclosed that the alloy may further contain up to 1% silver and up to 1% germanium. However, no examples are given regarding the addition of Ag or Ge, nor is any effect thereof disclosed.